The present application relates generally to output stages of amplifier circuits, and more specifically to a method of compensating for nonlinearity introduced by the dead time in the switching output stage of a class D amplifier.
The output stage of a conventional class D amplifier typically includes two MOSFET transistors serially connected between upper and lower terminals of a power supply. The two transistors operate as switches to commute the output of the amplifier between the voltage levels at the upper and lower power supply terminals, e.g., VP and GND. Specifically, when an output transistor is switched “on”, the voltage across the transistor is small, e.g., close to 0 volts. Further, when an output transistor is switched “off”, the current through the transistor is essentially zero. As a result, the voltage level at the amplifier output can be approximately VP or GND, depending upon which output transistor is switched on and which one is switched off.
To avoid a large and potentially harmful flow of current between the power supply terminals, the class D amplifier operates such that the two output transistors are not switched on at the same time. Specifically, a predefined non-zero delay or “dead time” is provided from the time one output transistor is switched off to the time the other output transistor is switched on. During this dead time, both output transistors are switched off, and the output of the amplifier is tri-stated, i.e., the amplifier output is in a high impedance state.
The above-described conventional class D amplifier output stage has drawbacks because, in practice, the body diodes of the MOSFET output transistors typically allow a residual load current to flow through the transistors when they are switched off. The direction of this residual load current can define the state of the amplifier output during the dead time in the switching of the output stage. Further, because it is not directly related to the input of the amplifier, the residual load current can cause the duty cycle of the amplifier output to be different from the duty cycle of the amplifier input. As a result, nonlinearity may be introduced in the amplifier output, which can adversely affect specified amplifier performance characteristics such as total harmonic distortion (THD).
It would therefore be desirable to have an improved method of operating the output stage of a class D amplifier that avoids the drawbacks of the above-described conventional class D amplifier output stage.